In scanning tunneling microscopy, a probe tip is brought close to a sample so that a space of approximately a few nanometers (nm) may be left between them, in order that an electron cloud on the tip and an electron cloud on the sample surface may be superimposed. Under this condition, a voltage is applied between the tip and the sample. The resulting current is called a tunneling current. When the applied voltage is several millivolts (mv) to several volts (V), the tunneling current is approximately 1 to 10 nanoamperes (nA). The magnitude of the tunneling current is proportional to the distance between the sample and the tip. Thus, this distance can be determined quite accurately by measuring the magnitude of the tunneling current. The tip is scanned across the sample surface at a constant height above a reference plane in the sample. The produced tunneling current is measured to determine the topography of the sample surface. If the position of the tip is so controlled that the tunneling current is kept constant, then the topography of the sample surface can be similarly determined by tracing the position of the tip. The principle of the scanning tunneling microscope is explained in U.S. Pat. No. 4,343,993.
The scanning tunneling microscope has very high resolution, but its field of view is quite narrow. Therefore, it is impossible to know from an image obtained by the scanning tunneling microscope what portion of the sample is being observed. In other words, it is important to decide what portion of the sample should be observed with the scanning tunneling microscope to make a topographical analysis of the sample. Using a scanning electron microscope as a means for determining the field of view of a scanning tunneling microscope has been reported. One example of a scanning electron microscope incorporating a scanning tunneling microscope is described in an article entitled "Scanning Tunneling Microscope Combined With a Scanning Electron Microscope" by Ch. Gerber, G. Binning, H. Fuchs, O. Marti, and H. Rohrer in Rev. Sci. Instrum. 57(2), February 1986, pp. 221-224.
Greatly uneven sample surfaces can be observed very well by scanning electron microscopy. However, it is difficult using scanning electron microscopy to make a topographical analysis of a sample surface that is only slightly uneven, because its protruding portions and recessed portions emit secondary electrons at close rates. Consequently, scanning electron microscopy is not very suitable for locating a flat surface that is necessary for the scanning tunneling microscope.
It is therefore desirable to use a transmission electron microscope having higher resolution than a scanning electron microscope to determine the field of view of a scanning tunneling microscope. Because the transmission electron microscope has higher resolution than the scanning electron microscope, and therefore a small gap between the objective lens pole pieces, the smallness of the gap must be taken into account in mounting a scanning tunneling microscope into a transmission electron microscope. Also, the scanner of the scanning tunneling microscope must be so mounted in the gap that it is insusceptible to vibration.